In order to meet an ever increasing demand for data services in cellular mobile applications, wireless communication standards characterised by high spectral efficiency and making use of a spatial dimension in transmitting information between base stations and mobile terminals are defined and the requirement for testing the performance characteristics of devices operable under such standards increases.
Orthogonal frequency division multiplexing (OFDM) is one of the spectrally efficient transmission schemes, being employed for example in wireless communication under the standards long term evolution (LTE) for a fourth generation cellular wireless system or some of the IEEE 802.11 series for wireless local area networks (WLAN).
Encoded bit streams to be transmitted are divided into plural data streams with reduced bit rate, the bit information is mapped onto constellation symbols for transmission. Then each partial data stream is modulated onto a carrier signal, wherein the carrier frequencies are orthogonal to each other, the modulated carrier signals comprising the constellation symbols of said partial data streams are added to form a transmission signal and converted to a transmission frequency.
An improvement in transmission efficiency is further achieved by employing multiple antennas both at base stations and wireless devices, thereby defining multiple input multiple output systems (MIMO). An encoded bit stream for transmission may be distributed onto different antennas of the transmitting device as sources and transmitted on the same frequency to a receiving device as destination. Hence multiple different transmission paths, either direct paths or indirect paths including reflected transmission signals reduce losses being inflicted due to superposition of signals received via multiple signal path propagation. Hence increased data rates at an advantageous receive signal quality are achieved by employing MIMO techniques.
Testing of devices for multicarrier systems and even more when also testing of MIMO capabilities is concerned, requires dedicated test equipment for reproducible tests under realistic test conditions. The external influences on signal transmission in a real world communication system between a base station as an example for a signal source and the device under test (DUT) are simulated by a signal generator and a channel simulator. In known test systems the channel simulator modifies the test signal generated by a signal generator in the time domain according to given channel characteristics and provides the modified signal to the DUT. The computational requirements for executing the required calculations in the channel simulator are large particularly when taking multicarrier systems into account.
DE 10 2008 055 759 A1 proposes to integrate a channel simulator unit into a test device for testing a mobile communication terminal. The channel simulation is implemented by distorting one or more of the modulated carrier signals by employing a static and frequency selective channel model in the frequency domain. Time variant effects on the transmission signal are excluded by the test device of DE 10 2008 055 759 A1 and in particular examination of fading effects on test signals are therefore not sufficiently addressed in the cited reference.